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United States Patent |
6,172,589
|
Fujita
,   et al.
|
January 9, 2001
|
Hard magnetic alloy having supercooled liquid region, sintered or cast
product thereof or stepping motor and speaker using the alloy
Abstract
A hard magnetic alloy obtained by heat treatment, at a heating rate of
20.degree. C./min or more, of a glassy alloy containing Fe as a main
component, at least one element R selected from the rare earth elements,
at least one selected from Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, and Cu, and
B, and having a supercooled liquid region having a temperature width
.DELTA.Tx of 20.degree. C. or more, which is represented by the equation
.DELTA.Tx=Tx-Tg (wherein Tx indicates the crystallization temperature, and
Tg indicates the glass transition temperature), and a sintered compact, a
cast magnet, a stepping motor and a speaker each of which includes the
hard magnetic alloy.
Inventors:
|
Fujita; Kouichi (Miyagi-ken, JP);
Makino; Akihiro (Niigata-ken, JP);
Inoue; Akihisa (Miyagiken, JP)
|
Assignee:
|
Alps Electric Co., Ltd. (Tokyo, JP);
Akihisa Inoue (Miyagi-ken, JP)
|
Appl. No.:
|
138149 |
Filed:
|
August 21, 1998 |
Foreign Application Priority Data
| Aug 22, 1997[JP] | 9-226867 |
| Aug 28, 1997[JP] | 9-233068 |
| Aug 29, 1997[JP] | 9-235289 |
| Aug 29, 1997[JP] | 9-235290 |
| Aug 29, 1997[JP] | 9-249931 |
| Aug 29, 1997[JP] | 9-249935 |
Current U.S. Class: |
335/306; 335/302 |
Intern'l Class: |
H01F 007/02 |
Field of Search: |
145/104,304,101,538
335/302-6
|
References Cited
U.S. Patent Documents
5172751 | Dec., 1992 | Croat | 148/101.
|
5725684 | Mar., 1998 | Inoue et al. | 148/304.
|
5976274 | Nov., 1999 | Inoue et al. | 148/304.
|
Foreign Patent Documents |
0 542 529 A1 | May., 1993 | JP.
| |
06124820A | May., 1994 | JP.
| |
06124825A | May., 1994 | JP.
| |
0 632 471 A2 | Jan., 1995 | JP.
| |
0 867 897 | Sep., 1998 | JP.
| |
Other References
Koji Ma A et al. "Structural and Magnetic Properties of Nanocrystalline
Fe-Rich Fe-Nb-Nd-B Sintered Magnets Produced By Consolidating Amorphous
Powders" IEEE Transactions On Magnetics, vol. 33, No. 5, part 02, Sep.
1997, pp. 3817-3819; XP000703227 Intermag Conference, New Orleans Apr.
1997.
Patent Abstracts of Japan, vol. 016, No. 423 (E-1260), Sep. 7, 1992 & JP 04
147605A (Hitachi Metals Ltd), May 21, 1992.
Abstract, Database WPI,Section Ch, Week 9536, Derwent Publications ltd.,
London, GB; Class L03, AN 95-273107 XP002092150 & JP 07 173501 A (Sumitomo
Special Metals Co Ltd), Jul. 11, 1995.
|
Primary Examiner: Donovan; Lincoln
Attorney, Agent or Firm: Brinks Hofer Gilson & Lione
Claims
What is claimed is:
1. A hard magnetic alloy having soft and hard magnetic phases, the hard
magnetic alloy obtained by heat treatment of a glassy alloy at a heating
rate of 20.degree. C./min or more, wherein the glassy alloy contains Fe as
a main component, at least one element R selected from the rare earth
elements, the content of the at least one element R being no greater than
15 atomic %, at least one element M selected from Ti, Zr, Hf, V, Nb, Ta,
Cr, Mo, W, and Cu, and B, and having a supercooled liquid region having a
temperature width .DELTA.Tx of 20.degree. C. or more, which is represented
by the equation .DELTA.Tx=Tx-Tg, wherein Tx indicates the crystallization
temperature, and Tg indicates a glass transition temperature.
2. A hard magnetic alloy according to claim 1, wherein a crystalline phase
composed of one or both of an .alpha.-Fe phase and a Fe.sub.3 B phase, and
a crystalline phase composed of a Nd.sub.2 Fe.sub.14 B phase are
precipitated in the hard magnetic alloy.
3. A hard magnetic alloy according to claim 1 represented by the following
composition formula:
Fe.sub.100-x-y-z-w R.sub.x M.sub.y T.sub.z B.sub.w
wherein T is at least one element selected from Co and Ni, and the
composition ratios x, y, z and w by atomic % satisfy 2 atomic
%.ltoreq.x.ltoreq.15 atomic %, 2 atomic %.ltoreq.y.ltoreq.20 atomic %,
0.ltoreq.z.ltoreq.20 atomic %, and 10 atomic %.ltoreq.w.ltoreq.30 atomic
%, respectively.
4. A hard magnetic alloy according to claim 3, wherein a crystalline phase
composed of one or both of a .alpha.-Fe phase and a Fe.sub.3 B phase, and
a crystalline phase composed of a Nd.sub.2 Fe.sub.14 B phase are
precipitated in the hard magnetic alloy.
5. A hard magnetic alloy according to claim 1 represented by the following
composition formula:
Fe.sub.100-x-y-z-w-t R.sub.x M.sub.y T.sub.z B.sub.w L.sub.t
wherein T is at least one element selected from Co and Ni, the composition
ratios x, y, z, w and t by atomic % satisfy 2 atomic % .ltoreq.x.ltoreq.15
atomic %, 2 atomic % .ltoreq.y.ltoreq.20 atomic %, 0.ltoreq.z.ltoreq.20
atomic %, 10 atomic % .ltoreq.w.ltoreq.30 atomic %, and
0.ltoreq.t.ltoreq.5 atomic %, respectively, and L is at least one element
selected from Ru, Rh, Pd, Os, Ir, Pt, Al, Si, Ge, Ga, Sn, C and P.
6. A hard magnetic alloy according to claim 5, wherein a crystalline phase
composed of one or both of a .alpha.-Fe phase and a Fe.sub.3 B phase, and
a crystalline phase composed of a Nd.sub.2 Fe.sub.14 B phase are
precipitated in the hard magnetic alloy.
7. A hard magnetic alloy having soft and hard magnetic phases, the hard
magnetic alloy sintered compact obtained by sintering a glassy alloy
powder under heat and pressure, wherein the glassy alloy powder contains
Fe as a main component, at least one element R selected from the rare
earth elements, the content of the at least one element R being no greater
than 15 atomic %, at least one element M selected from Ti, Zr, Hf, V, Nb,
Ta, Cr, Mo, W, and Cu, and B, and having a supercooled liquid region
having a temperature width .DELTA.Tx of 20.degree. C. or more, which is
represented by the equation .DELTA.Tx=Tx-Tg, wherein Tx indicates the
crystallization temperature, and Tg indicates a glass transition
temperature.
8. A hard magnetic alloy sintered compact according to claim 7, wherein the
glassy alloy has the composition represented by the following composition
formula:
Fe.sub.100-x-y-z-w R.sub.x M.sub.y T.sub.z B.sub.w
wherein R is the rare earth element, M is the metal M, T is at least one
element selected from Co and Ni, and the composition ratios x, y, z and w
by atomic % satisfy 2.ltoreq.x.ltoreq.15, 2.ltoreq.y.ltoreq.20,
0.ltoreq.z.ltoreq.20, and 10.ltoreq.w.ltoreq.30, respectively.
9. A hard magnetic alloy sintered compact according to claim 7, wherein the
glassy alloy has the composition represented by the following composition
formula:
Fe.sub.100-x-y-z-w-t R.sub.x M.sub.y T.sub.z B.sub.w L.sub.t
wherein R is the rare earth element, M is the metal M, T at least one
element selected from Co and Ni, L is at least one element selected from
Ru, Rh, Pd, Os, Ir, Pt, Al, Si, Ge, Ga, Sn, C and P, and the composition
ratios x, y, z, w and t by atomic % satisfy 2.ltoreq.x.ltoreq.15,
2.ltoreq.y.ltoreq.20, 0.ltoreq.z.ltoreq.20, 10.ltoreq.w.ltoreq.30,and
0.ltoreq.t.ltoreq.5, respectively.
10. A cast magnet having soft and hard magnetic phases, the cast magnet
obtained by casting a glassy alloy composition and then performing heat
treatment, where the glassy alloy composition contains Fe as a main
component, at least one element R selected from the rare earth elements,
the content of the at least one element R being no greater than 15 atomic
%, at least one element M selected from Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W,
and Cu, and B, and having a supercooled liquid region having a temperature
width .DELTA.Tx of 20.degree. C. or more, which is represented by the
equation .DELTA.Tx=Tx-Tg, wherein Tx indicates the crystallization
temperature, and Tg indicates a glass transition temperature.
11. A cast magnet according to claim 10, wherein the glassy alloy has the
composition represented by the following composition formula:
Fe.sub.100-x-y-z-w R.sub.x M.sub.y T.sub.z B.sub.w
wherein R is the rare earth element, M is the metal M, T is at least one
element selected from Co and Ni, and the composition ratios x, y, z and w
by atomic % satisfy 2 .ltoreq.x.ltoreq.15, 2.ltoreq.y.ltoreq.20,
0.ltoreq.z.ltoreq.20, and 10.ltoreq.w.ltoreq.30, respectively.
12. A cast magnet according to claim 10, wherein the glassy alloy has the
composition represented by the following composition formula:
Fe.sub.100-x-y-z-w R.sub.x M.sub.y T.sub.z B.sub.w
wherein R is the rare earth element, M is the metal M, T is at least one
element selected from Co and Ni, L is at least one element selected from
Ru, Rh, Pd, Os, Ir, Pt, Al, Si, Ge, Ga, Sn, C and P, and the composition
ratios x, y, z, w and t by atomic % satisfy 2.ltoreq.x.ltoreq.15,
2.ltoreq.y.ltoreq.20, 0.ltoreq.z.ltoreq.20, 10.ltoreq.w.ltoreq.30, and
0.ltoreq.t.ltoreq.5, respectively.
13. A hard magnetic alloy according to claim 1, wherein heat treatment of
the glassy alloy is at a heating rate of 20.degree. C./min or more and
less than 80.degree. C./min.
14. A hard magnetic alloy according to claim 13, wherein the glassy alloy
has composition Fe.sub.63 Co.sub.7 Nd.sub.6 Cr.sub.4 B.sub.20.
15. A hard magnetic alloy according to claim 1, wherein the hard magnetic
alloy forms a rotor for a stepping motor.
16. A hard magnetic alloy according to claim 1, wherein the hard magnetic
alloy forms a speaker magnet.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a hard magnetic alloy having a supercooled
liquid region, and particularly to a hard magnetic alloy which has
excellent hard magnetism at room temperature and which can be formed to a
bulk permanent magnet comprising a sintered or cast product. The present
invention also relates to a stepping motor and a speaker using the hard
magnetic alloy.
2. Description of the Related Art
Some types of multi-element alloys are conventionally known to have a wide
supercooled liquid region before crystallization and constitute glassy
alloys. It is also known that such glassy alloys can be formed to bulk
alloys significantly thicker than amorphous alloy ribbons produced by a
conventional known liquid quenching method.
Examples of conventional known amorphous alloy ribbons include ribbons of
Fe--P--C system amorphous alloys first produced in the 1960's,
(Fe,Co,Ni)--P--B system and (Fe,Co,Ni)--Si--B system alloys produced in
the 1970's, and (Fe,Co,Ni)--M (Zr,Hf,Nb) system alloys and (Fe,Co,Ni)--M
(Zr,Hf,Nb)--B system alloys produced in the 1980's. All these alloys must
be produced by quenching at a cooling rate in the 10.sup.-5.degree. C./s
level, and the produced ribbons have a thickness of 50 .mu.m or less.
On the other hand, glassy alloys having a thickness of several millimeters
are obtained, and such glassy alloys include alloys having the Ln--Al--TM,
Mg--Ln--TM and Zr--Al--TM (wherein Ln represents a rare earth element, and
TM represents a transition metal) system compositions and the like, which
were discovered in 1988 to 1991.
However, all these conventional known glassy alloys have no magnetism at
room temperature, and from this viewpoint, these alloys are industrially
greatly restricted when considered as hard magnetic materials.
Therefore, if a thick alloy comprising an amorphous single phase is
obtained, the crystal structure is made fine and uniform after heat
treatment, and good magnetic properties are expected. Therefore, research
and development have conventionally proceeded with respect to glass alloys
having hard magnetism at room temperature and permitting the formation of
thick bulk products. These alloys having various compositions exhibit a
supercooled liquid crystal liquid state at room temperature, but the
temperature width .DELTA.Tx of the supercooled liquid region, i.e., the
difference (Tx-Tg) between the crystallization temperature (Tx) and the
glass transition temperature (Tg), is generally small. Therefore, in fact,
such alloys have the low ability to form glassy alloys, and are thus
unpractical. In consideration of this, the presence of an alloy having a
supercooled liquid region having a wide temperature width, and capable of
forming a glassy alloy by cooling overcomes thickness restrictions of
conventional known amorphous alloy ribbons, and such an alloy thus
attracts much attention in the metallurgical field. However, whether or
not such an alloy can be developed as an industrial material depends upon
the finding of a glassy alloy exhibiting ferromagnetism at room
temperature.
Also examples of conventional known magnet materials having performance
superior to ferrite magnets include Sm--Co sintered magnets, Fe--Nd--B
sintered magnets, Fe--Nd--B quenched magnets, and the like. In order to
achieve higher performance, there are many researches on new alloy magnets
such as Fe--Sm--N magnets.
However, these magnet materials must contain 10 atomic % or more of Nd or 8
atomic % or more of Sm, and thus have the drawback that the production
cost is higher than the ferrite magnets because a large amount of
expensive rare earth element is used. The ferrite magnets are produced at
lower cost than these rare earth magnets, but have insufficient magnetic
properties. Therefore, there is demand for appearance of a magnet material
costing less and exhibiting hard magnetism higher than ferrite magnets.
On the other hand, as a magnet generally known as a "bonded magnet", a
magnet formed by compression molding or injection molding a mixture of a
magnetic powder and a rubber or plastic binder can widely be used as
electronic parts because of high shape freedom, but has the problems of
low magnetic performance due to low remanent magnetization, and low
material strength because of inclusion of the binder.
Possible applications of these magnets include a stepping motor and a
speaker.
The stepping motor is a special motor in which the rotation can be
arbitrarily controlled by a pulse current. Therefore, the stepping motor
requires no feedback control, is capable of positioning in an open loop,
and used as a drive source in a positioning control system in various
fields. Particularly, since a hybrid stepping motor has high rotational
torque and is small and capable of performing precise positioning control,
the hybrid stepping motor is used as a drive source for a driving
mechanism in a copying machine, a computer, or the like.
The characteristics of a hard magnetic alloy are represented by the second
quadrants of hysteresis curves, i.e., demagnetization curves. After
magnetization, a hard magnetic alloy is under the reverse magnetic field,
i.e., the diamagnetic field, produced by its remanent magnetization, and
thus the operating point (the magnetic flux density (B) and demagnetizing
field (H) of a material) is represented by a point p on the
demagnetization curve thereof. At this point, the product (BH) represents
the maximum energy product ((BH).sub.max).
In order to increase the rotational torque of the stepping motor, it is
important to use a hard magnetic alloy having the high maximum energy
product ((BH).sub.max).
Since the rotational torque of the stepping motor is proportional to the
product of the current passing through the stepping motor and the energy
(U) of the magnetostatic field produced outside by a hard magnetic alloy,
the rotational torque of the stepping motor is increased by increasing the
maximum energy product ((BH).sub.max).
In order to increase the maximum energy product ((BH).sub.max) of a hard
magnetic alloy, it is necessary to made the shape of a demagnetization
curve angular to increase the area surrounded by the demagnetization
curve, the magnetic field axis and the magnetization axis. Namely, it is
necessary to increase the remanence ratio (Ir/Is) to increase remanent
magnetization (Ir) and coercive force (iHc).
Therefore, as a hard magnetic alloy used for a rotor of a HB type motor, a
Al--Ni--Co--Fe system magnet, a Nd--Fe--B system sintered magnet, a
Nd--Fe--B type bonded magnet, a Sm--Co system sintered magnet, or the like
is used.
However, in a stepping motor using a Al--Ni--Co--Fe system magnet, since
the Al--Ni--Co--Fe system magnet has a coercive force (iHc) of as low as 1
kOe or less, there is the problem of causing difficulties in attempting to
decease the size of the stepping motor.
Although a Nd--Fe--B system sintered magnet and Sm--Co system sintered
magnet have high coercive force (iHc) and are thus used for some of small
stepping motors, these magnets have the need to sinter a material powder
in the production process, and thus have the problem of increasing the
production cost of a magnet, thereby increasing the production cost of a
stepping motor.
Furthermore, a Nd--Fe--B system bonded magnet is produced by mixing a
rubber or plastic binder with a magnetic powder formed by liquid quenching
of an alloy melt mainly comprising the Nd.sub.2 Fe.sub.14 B phase or a
Fe.sub.3 B--Nd.sub.2 Fe.sub.14 system exchange spring magnetic powder and
then compressing molding or injection molding, and thus has low material
strength because of inclusion of the binder. There is thus a problem in
that the rotor of a stepping motor serving as a driving unit has low
strength.
Also, from the viewpoint of material strength, a ribbon having a thickness
of about 50 .mu.m or less and obtained by quenching a melt of a Nd--Fe--B
system alloy is preferable from the viewpoint of mechanical strength.
However, in the use of such a hard magnetic alloy ribbon as the rotor of a
stepping motor, many ribbons must be laminated, thereby causing the
problem of increasing the production cost of the stepping motor.
A conventional known speaker schematically comprises a pole piece made of
iron, a cylindrical yoke provided on the outside of the pole piece with a
space therebetween, upper and lower speaker magnet rings provided in the
space between the pole piece and the yoke, and a conical diaphragm. In
addition, a voice coil is provided in the magnetic gap formed by the
speaker magnets, the voice coil being connected to the conical diaphragm.
In such a speaker, when a voice current flows from an amplifier to the
voice coil, motion accordingly occurs to move the conical diaphragm
connected to the voice coil so that sounds can be emitted.
In a conventional speaker, a ferrite magnet or a Al--Ni--Co--Fe system
magnet is used as a speaker magnet material, and a Nd--Fe--B system magnet
or a Sm--Co system magnet is used as a magnet material having performance
superior to the ferrite magnet and Al--Ni--Co--Fe system magnet.
Furthermore, many researches have been made for achieving higher
performance by using new alloy magnets such as a Sm--Fe--N system alloy
and the like.
However, as described above, the Nd--Fe--B system magnet, the Sm--Co system
magnet and the Sm--Fe--N system magnet require 10 atomic % or more of Nd
or 8 atomic % or more of Sm, and thus have a fault that the production
cost is higher than a ferrite magnet and Al--Ni--Co--Fe system magnet
because a large amount of expensive rare earth element is used. In
addition, the Sm--Co system magnet is a more expensive magnet than the
Nd--Fe--B system magnet, and is thus impractical. On the other hand, the
Al--Ni--Co--Fe system magnet costs less than a rare earth magnet, but has
the problem of excessively low coercive force. There is thus demand for
appearance of a speaker magnet material which costs less and has higher
hard magnetic properties than a ferrite magnet.
SUMMARY OF THE INVENTION
Accordingly, a first object of the present invention is to provide a hard
magnetic alloy which has a supercooled liquid region having a wide
temperature width .DELTA.Tx, excellent hard magnetism at room temperature,
and excellent strength as a material, and which can be formed to a thicker
shape than an amorphous alloy ribbon obtained by a conventional liquid
quenching method.
A second object of the present invention is to provide a hard magnetic
alloy molding having excellent strength as a material and excellent
magnetic performance.
A third object of the present invention is to provide a cast magnet, which
has excellent material strength and magnetic performance and high shape
freedom, and which can be used as a permanent magnet member.
A fourth object of the present invention is to provide a stepping motor
which permits miniaturization, which comprises a rotor having high
strength, and which can be produced at low cost.
A fifth object of the present invention is to obtain a hard magnetic alloy
which has a supercooled liquid region having a wide temperature width
.DELTA.Tx, excellent hard magnetism at room temperature, and excellent
strength as a material, which can be formed to a thicker shape than an
amorphous alloy ribbon obtained by a conventional liquid quenching method,
and which costs less than a rare earth magnet, and to provide a speaker
using the hard magnetic alloy as a speaker magnet material.
The hard magnetic alloy having a supercooled liquid region according to the
present invention is obtained by heat treatment of a glassy alloy at a
heating rate of 20.degree. C./min or more, wherein the glassy alloy
comprises Fe as a main component, at least one element R selected from the
rare earth elements, at least one element M selected from Ti, Zr, Hf, V,
Nb, Ta, Cr, Mo, W and Cu, and B, and has a supercooled liquid region
having a temperature width .DELTA.Tx of 20.degree. C. or more represented
by the equation .DELTA.Tx=Tx-Tg (wherein Tx is the crystallization
temperature, and Tg is the glass transition temperature).
The hard magnetic alloy having a supercooled liquid region according to the
present invention may be represented by the following formula:
Fe.sub.100-x-y-z-w R.sub.x M.sub.y T.sub.z B.sub.w
wherein T is at least one element selected from Co and Ni, and the
composition ratios x, Y, Z and w by atomic % satisfy 2 atomic
%.ltoreq.x.ltoreq.15 atomic %, 2 atomic %.ltoreq.y.ltoreq.20 atomic %,
0.ltoreq.z.ltoreq.20 atomic %, and 10 atomic %.ltoreq.w.ltoreq.30 atomic
%, respectively.
The hard magnetic alloy having a supercooled liquid region according to the
present invention may be represented by the following formula:
Fe.sub.100-x-y-z-w-t R.sub.x M.sub.y T.sub.z B.sub.w L.sub.t
wherein T is at least one element selected from Co and Ni, the composition
ratios x, Y, Z, w and t by atomic % satisfy 2 atomic %.ltoreq.x.ltoreq.15
atomic %, 2 atomic %.ltoreq.y.ltoreq.20 atomic %, 0.ltoreq.z.ltoreq.20
atomic %, 10 atomic %.ltoreq.w.ltoreq.30 atomic %, and 0.ltoreq.t.ltoreq.5
atomic %, respectively, and L is at least one element selected from Ru,
Rh, Pd, Os, Ir, Pt, Al, Si, Ge, Ga, Sn, C and P.
In the present invention, the hard magnetic alloy subjected to heat
treatment may comprise a crystalline phase composed of at least one of a
.alpha.-Fe phase and Fe.sub.3 B phase, and a crystalline phase composed of
a Nd.sub.2 Fe.sub.14 B phase, both of which are precipitated.
In the present invention, even if the hard magnetic alloy contains small
amounts of impurities inevitable in the production process, for example,
rare earth oxides, and the like, it can be considered as lying within the
scope of the technical idea of the hard magnetic alloy of the present
invention.
In order to achieve the objects, the present invention provides a hard
magnetic alloy sintered compact obtained by sintering, under heat and
pressure, a glassy alloy powder comprising Fe as a base metal, at least
one rare earth element R, at least one metal M selected from Ti, Zr, Hf,
V, Nb, Ta, Cr, Mo, W and Cu, and B, and having a supercooled liquid region
having a temperature width .DELTA.Tx of 20.degree. C. or more represented
by the equation .DELTA.Tx=Tx-Tg (wherein Tx is the crystallization
temperature, and Tg is the glass transition temperature), and at the same
time, imparting magnetic anisotropy to the glassy alloy powder.
The glassy alloy preferably has the composition represented by the
following formula (1):
Fe.sub.100-x-y-z-w R.sub.x M.sub.y T.sub.z B.sub.w Formula (1)
wherein R is the rare earth element, T is at least one element selected
from Co and Ni, and the composition ratios x, Y, Z and w by atomic %
satisfy 2 atomic %.ltoreq.x.ltoreq.15 atomic %, 2 atomic %
.ltoreq.y.ltoreq.20 atomic %, 0.ltoreq.z.ltoreq.20 atomic %, and 10 atomic
%.ltoreq.w.ltoreq.30 atomic %, respectively. Also element L (at least one
element selected from Ru, Rh, Pd, Os, Ir, Pt, Al, Si, Ge, Ga, Sn, C and P)
may be added up to an upper limit of 5 atomic %.
In production of the hard magnetic alloy sintered compact, the present
invention also provides a method of producing the hard magnetic alloy
sintered compact comprising sintering molding the glassy alloy under heat
and pressure by a spark plasma sintering process, and at the same time
imparting magnetic anisotropy to the glassy alloy powder. At this time,
the sintering temperature Ts is preferably in the range of the
crystallization temperature Tx.+-.250 (K), and the sintering pressure is
preferably in the range of 200 to 1500 MPa.
In order to achieve the objects, the present invention provides a cast
magnet obtained by casting a glassy alloy composition comprising Fe as a
base metal, at least one rare earth element R, at least one metal M
selected from Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W and Cu, and B, and having a
supercooled liquid region having a temperature width .DELTA.Tx of
20.degree. C. or more represented by the equation .DELTA.Tx=Tx-Tg (wherein
Tx is the crystallization temperature, and Tg is the glass transition
temperature), and then performing heat treatment to precipitate a soft
magnetic phase and a hard magnetic phase.
The glassy alloy composition preferably has the composition represented by
the following formula (1):
Fe.sub.100-x-y-z-w R.sub.x M.sub.y T.sub.z B.sub.w Formula (1)
wherein R is the rare earth element, M is the metal, T is at least one
element selected from Co and Ni, and the composition ratios x, Y, Z and w
by atomic % satisfy 2 atomic %.ltoreq.x.ltoreq.15 atomic %, 2 atomic
%.ltoreq.y.ltoreq.20 atomic %, 0.ltoreq.z.ltoreq.20 atomic %, and 10
atomic %.ltoreq.w.ltoreq.30 atomic %, respectively. Also element L (at
least one element selected from Ru, Rh, Pd, Os, Ir, Pt, Al, Si, Ge, Ga,
Sn, C and P) may be added up to an upper limit of 5 atomic %.
The present invention also provides a method of producing the cast magnet
comprising melting the glassy alloy composition, casting the resultant
melt to form a solidified product of the glassy alloy, and then performing
heat treatment to precipitate a soft magnetic phase and a hard magnetic
phase. The heat treatment of the glassy alloy solidified product obtained
by casting is preferably carried out at a temperature in the range of 500
to 850.degree. C.
In order to achieve the objects, the present invention further employs the
following construction.
A stepping motor of the present invention comprises a stator comprising an
electromagnet, and a rotor comprising a hard magnetic alloy having a
supercooled liquid region, wherein the hard magnetic alloy comprises Fe as
a main component, at least one element R selected from the rare earth
elements, at least one element M selected from Ti, Zr, Hf, V, Nb, Ta, Cr,
Mo, W and Cu, and B, and has a supercooled liquid region having a
temperature width .DELTA.Tx of 20 K or more represented by the equation
.DELTA.Tx=Tx-Tg (wherein Tx is the crystallization temperature, and Tg is
the glass transition temperature).
In the above-mentioned stepping motor of the present invention, the hard
magnetic alloy having a supercooled liquid region may have the composition
represented by the following formula:
Fe.sub.100-x-y-z-w R.sub.x M.sub.y T.sub.z B.sub.w
wherein T is at least one element selected from Co and Ni, and the
composition ratios x, Y, Z and w by atomic % satisfy 2.ltoreq.x.ltoreq.15,
2.ltoreq.y.ltoreq.20, 0.ltoreq.z.ltoreq.20, and 10.ltoreq.w.ltoreq.30,
respectively.
In the above-mentioned stepping motor of the present invention, the hard
magnetic alloy having a supercooled liquid region may have the composition
represented by the following formula:
Fe.sub.100-x-y-z-w-t R.sub.x M.sub.y T.sub.z B.sub.w L.sub.t
wherein T is at least one element selected from Co and Ni, L is at least
one element selected from Ru, Rh, Pd, Os, Ir, Pt, Al, Si, Ge, Ga, Sn, C
and P, and the composition ratios x, Y, Z, w and t by atomic % satisfy
2.ltoreq.x.ltoreq.15 , 2.ltoreq.y.ltoreq.20, 0.ltoreq.z.ltoreq.20,
10.ltoreq.w.ltoreq.30, and 0.ltoreq.t.ltoreq.5, respectively.
In the above-described stepping motor of the present invention, the hard
magnetic alloy is obtained by heat treatment and comprises the
precipitated crystalline phase composed of at least one of a .alpha.-Fe
phase and Fe.sub.3 B phase, and the precipitated crystalline phase
composed of a Nd.sub.2 Fe.sub.14 B phase.
A speaker of the present invention comprises a speaker magnet comprising a
hard magnetic alloy containing Fe as a main component, at least one
element R selected from the rare earth elements, at least one element M
selected from Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W and Cu, and B, and having a
supercooled liquid region having a temperature width .DELTA.Tx of
20.degree. C. or more represented by the equation .DELTA.Tx=Tx-Tg (wherein
Tx is the crystallization temperature, and Tg is the glass transition
temperature).
The hard magnetic alloy having a supercooled liquid region may be
represented by the following composition formula:
Fe.sub.100-x-y-z-w R.sub.x M.sub.y T.sub.z B.sub.w
wherein T is at least one element selected from Co and Ni, and the
composition ratios x, Y, Z and w by atomic % satisfy 2.ltoreq.x.ltoreq.15,
2.ltoreq.y.ltoreq.20, 0.ltoreq.z.ltoreq.20, and 10.ltoreq.w.ltoreq.30,
respectively.
The hard magnetic alloy having a supercooled liquid region may be
represented by the following composition formula:
Fe.sub.100-x-y-z-w-t R.sub.x M.sub.y T.sub.z B.sub.w L.sub.t
wherein T is at least one element selected from Co and Ni, L is at least
one element selected from Ru, Rh, Pd, Os, Ir, Pt, Al, Si, Ge, Ga, Sn, C
and P, and the composition ratios x, Y, Z, w and t by atomic % satisfy
2.ltoreq.x.ltoreq.15 , 2.ltoreq.y.ltoreq.20, 0.ltoreq.z.ltoreq.20,
10.ltoreq.w.ltoreq.30, and 0.ltoreq.t.ltoreq.5, respectively.
The hard magnetic alloy having a supercooled liquid region may be subjected
to heat treatment and comprise the precipitated crystalline phase composed
of at least one of a .alpha.-Fe phase and Fe.sub.3 B phase, and the
precipitated crystalline phase composed of a Nd.sub.2 Fe.sub.14 B phase.
In the present invention, even if the hard magnetic alloy contains small
amounts of impurities inevitable in the production process, for example,
rare earth oxides, and the like, it can be considered as lying within the
scope of the technical idea of the hard magnetic alloy having a
supercooled liquid region of the present invention.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a sectional view showing the structure of a principal portion of
an example of spark plasma sintering apparatus used for carrying out a
method of producing a hard magnetic alloy sintered compact of the present
invention;
FIG. 2 is a drawing showing an example of the waveforms of pulse currents
applied to an alloy powder in the spark plasma sintering apparatus shown
in FIG. 1;
FIG. 3 is a front view showing the entire construction of an example of
spark plasma sintering apparatus used for carrying out a method of
producing a hard magnetic alloy sintered compact of the present invention;
FIG. 4 is a schematic drawing showing an example of casting apparatus used
for producing a cast magnet of the present invention;
FIG. 5 is a schematic drawing showing the state wherein the casting
apparatus shown in FIG. 4 is used;
FIG. 6 is a schematic drawing showing another example of casting apparatus;
FIG. 7 is a perspective sectional view showing a hybrid stepping motor;
FIGS. 8A and 8B are drawings showing a rotor of a hybrid stepping motor, in
which FIG. 8A is a sectional plan view of the rotor, and FIG. 8B is a
schematic drawing showing the positional relation of the two rotor tooth
poles of the rotor;
FIGS. 9A and 9B are drawings illustrating the operation of a hybrid
stepping motor, in which FIG. 9A is an enlarged schematic drawing of a
rotor and a stator, and FIG. 9B is an enlarged schematic drawing of
another rotor and stator;
FIG. 10A and 10B are drawings illustrating the operation of a hybrid
stepping motor, in which FIG. 10A is an enlarged schematic drawing of a
rotor and a stator, and FIG. 10B is an enlarged schematic drawing of
another rotor and stator;
FIG. 11 is a sectional view showing a speaker in accordance with a first
embodiment of the present invention;
FIG. 12 is a sectional view showing a speaker in accordance with a second
embodiment of the present invention;
FIG. 13 is a diagram showing the results of measurement of DSC curves of
ribbon samples having the composition Fe.sub.63 Co.sub.7 Nd.sub.10-z
Zr.sub.x B.sub.20 (x=0, 2, 4 and 6 atomic %) after quenching in a single
roll production method;
FIG. 14 is a chart showing the result of X-ray diffraction analysis of a
ribbon sample having the composition Fe.sub.63 Co.sub.7 Nd.sub.6 Zr.sub.4
B.sub.20 after annealing at 560.degree. C. (833 K) for 300 seconds, and a
ribbon sample having the composition Fe.sub.63 Co.sub.7 Nd.sub.4 Zr.sub.6
B.sub.20 after annealing at 570.degree. C. (843 K) for 300 seconds;
FIG. 15 is a chart showing X-ray diffraction images of examples of glassy
alloys used as sintering raw material;
FIG. 16 is a graph showing the dependence of magnetic properties on the
heat treatment temperature with respect to ribbon samples having the
composition Fe.sub.63 Co.sub.7 Nd.sub.10-z Cr.sub.x B.sub.20 (x=0, 2, 4
and 6 atomic %) after heat treatment at 560 to 900.degree. C. for a
holding time of 300 seconds;
FIG. 17 is a graph showing the dependence of magnetic properties on the
heat treatment temperature with respect to a ribbon sample having the
composition Fe.sub.63 Co.sub.7 Nd.sub.6 Cr.sub.4 B.sub.20 after heat
treatment;
FIG. 18 is a graph showing the I-H loops of a ribbon sample having the
composition Fe.sub.63 Co.sub.7 Nd.sub.8 Cr.sub.2 B.sub.20 before and after
heat treatment;
FIG. 19 is a graph showing the I-H loops of a ribbon sample having the
composition Fe.sub.63 Co.sub.7 Nd.sub.6 Cr.sub.4 B.sub.20 before and after
heat treatment;
FIG. 20 is a graph showing the I-H loops of a ribbon sample having the
composition Fe.sub.63 Co.sub.7 Nd.sub.4 Cr.sub.6 B.sub.20 before and after
heat treatment;
FIG. 21 is a graph showing the I-H loops of a ribbon sample having the
composition Fe.sub.63 Co.sub.7 Nd.sub.10 B.sub.20 before and after heat
treatment;
FIG. 22 is a chart showing a DSC curve of a glassy alloy thin ribbon sample
having the composition Fe.sub.63 Co.sub.7 Nd.sub.6 Cr.sub.4 B.sub.20 ;
FIG. 23 is a chart showing a TMA curve and a DTMA curve of a glassy alloy
thin ribbon sample having the composition Fe.sub.63 Co.sub.7 Nd.sub.6
Cr.sub.4 B.sub.20 ; and
FIG. 24 is a graph showing a DSC curve of a glassy alloy solidified product
having the composition Fe.sub.63 Co.sub.7 Nd.sub.6 Cr.sub.4 B.sub.20.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Embodiments of the present invention will be described with reference to
the drawings.
A hard magnetic alloy having a supercooled liquid region of the present
invention is obtained by heat treatment of a glassy alloy at a rate of
temperature rise of 20.degree. C./min or more, wherein the glassy alloy
comprises Fe as a main component, at least one element R selected from the
rare earth elements, at least one element M selected from Ti, Zr, Hf, V,
Nb, Ta, Cr, Mo, W and Cu, and B, has a supercooled liquid region having a
temperature width .DELTA.Tx of 20.degree. C. or more represented by the
equation .DELTA.Tx=Tx-Tg (wherein Tx is the crystallization temperature,
and Tg is the glass transition temperature).
The above composition system preferably necessarily contains Cr because
.DELTA.Tx is 40.degree. C. or more.
An example of the hard magnetic alloy having a supercooled liquid region
according to the present invention may be represented by the following
formula:
Fe.sub.100-x-y-z-w R.sub.x M.sub.y T.sub.z B.sub.w
wherein T is at least one element selected from Co and Ni, and the
composition ratios x, Y, Z and w by atomic % preferably satisfy 2 atomic
%.ltoreq.x.ltoreq.15 atomic %, 2 atomic %.ltoreq.y.ltoreq.20 atomic %,
0.ltoreq.z.ltoreq.20 atomic %, and 10 atomic %.ltoreq.w.ltoreq.30 atomic
%, respectively.
Another example of the hard magnetic alloy having a supercooled liquid
region according to the present invention is represented by the following
formula:
Fe.sub.100-x-y-z-w-t R.sub.x M.sub.y T.sub.z B.sub.w L.sub.t
wherein T is at least one element selected from Co and Ni, the composition
ratios x, Y, Z, w and t by atomic % satisfy 2 atomic %.ltoreq.x.ltoreq.15
atomic %, 2 atomic %.ltoreq.y.ltoreq.20 atomic %, 0.ltoreq.z.ltoreq.20
atomic %, 10 atomic %.ltoreq.w.ltoreq.30 atomic %, and 0.ltoreq.t.ltoreq.5
atomic %, respectively, and L is at least one element selected from Ru,
Rh, Pd, Os, Ir, Pt, Al, Si, Ge, Ga, Sn, C and P.
In the present invention, in the composition formula Fe.sub.100-x-y-z-w
R.sub.x M.sub.y T.sub.z B.sub.w or Fe.sub.100-x-y-z-w-t R.sub.x M.sub.y
T.sub.z B.sub.w L.sub.t, the composition ratio x is preferably in the
range of 2 atomic %.ltoreq.x.ltoreq.12 atomic %, and more preferably in
the range of 2 atomic %.ltoreq.x.ltoreq.8 atomic %.
In the present invention, in the composition formula Fe.sub.100-x-y-z-w
R.sub.x M.sub.y T.sub.z B.sub.w or Fe.sub.100-x-y-z-w-t R.sub.x M.sub.y
T.sub.z B.sub.w L.sub.t, the composition ratio y is preferably in the
range of 2 atomic %.ltoreq.y.ltoreq.15 atomic %, and more preferably in
the range of 2 atomic %.ltoreq.y.ltoreq.6 atomic %.
In the present invention, in the composition formula Fe.sub.100-x-y-z-w
R.sub.x M.sub.y T.sub.z B.sub.w or Fe.sub.100-x-y-z-w-t R.sub.x M.sub.y
T.sub.z B.sub.w L.sub.t, the composition ratio z is preferably in the
range of 0.1 atomic %.ltoreq.z.ltoreq.20 atomic %, and more preferably in
the range of 2 atomic t.ltoreq.z.ltoreq.10 atomic %.
In the present invention, in the composition formula Fe.sub.100-x-y-z-w
R.sub.x M.sub.y T.sub.z B.sub.w or Fe.sub.100-x-y-z-w-t R.sub.x M.sub.y
T.sub.z B.sub.w L.sub.t, element M may be represented by (Cr.sub.1-a
M'.sub.a) wherein M' is at least one element selected from Ti, Zr, Hf, V,
Nb, Ta, Mo, W and Cu, and 0.ltoreq.a.ltoreq.1. Furthermore, in the hard
magnetic alloy represented by the above formula, the composition ratio a
is preferably in the range of 0.ltoreq.a.ltoreq.0.5.
In heat treatment in the present invention, heating is preferably performed
at a holding temperature of 500 to 850.degree. C., more preferably 550 to
750.degree. C., because the hard magnetic alloy having improved coercive
force and maximum energy product can be obtained. After heat treatment
(heating), the hard magnetic alloy is cooled by means such as water
quenching or the like.
From the viewpoint of improvements in the coercive force and maximum energy
product of the hard magnetic alloy, in the heat treatment, the rate of the
temperature rise to the holding temperature is 20.degree. C./min or more,
preferably 20 to 80.degree. C./min, and more preferably 40 to 80.degree.
C./min.
In the present invention, the above heat treatment of the glassy alloy in
the above composition system can cause precipitation of a crystalline
phase composed of at least one of an .alpha.-Fe phase and a Fe.sub.3 B
phase, and a crystalline phase composed of a Nd.sub.2 Fe.sub.14 B phase.
Since the hard magnetic alloy obtained by the heat treatment has a mixed
phase state comprising a soft magnetic phase composed of the precipitated
.alpha.-Fe phase and a hard magnetic phase composed of the precipitated
Nd.sub.2 Fe.sub.14 B phase, and thus exhibits the properties of an
exchange spring magnet in which the soft magnetic phase and hard magnetic
phase are magnetically coupled. In the present invention, the alloy having
.DELTA.Tx before heat treatment is considered as a glassy alloy and
differentiated from an amorphous alloy without .DELTA.Tx.
[Reason for Limiting the Composition]
In the composition system of the present invention, Fe as a main component
and Co are elements which bear magnetism and are important for obtaining a
high saturation magnetic flux density and excellent hard magnetic
properties.
In a composition system containing a large amount of Fe, .DELTA.Tx tends to
increase, and the value of .DELTA.Tx can be increased by appropriately
setting the Co content. The addition of combination with another element
permits an increase in the value of .DELTA.Tx without deterioration in
magnetic properties, and has the effect of increasing the Curie
temperature and decreasing the temperature coefficient.
Specifically, in order to securely obtain .DELTA.Tx, the composition ratio
z of element T is preferably in the range of 0.ltoreq.z.ltoreq.20 atomic
%, and in order to securely obtain a .DELTA.Tx of 20.degree. C. or more,
the composition ratio z of element T is preferably in the range of 2
atomic %.ltoreq.z.ltoreq.10 atomic %.
Co may be partially or entirely replaced by Ni according to demand.
R is at least one element selected from the rare earth elements (Y, La, Ce,
Pr, Nd, Gd, Tb, Dy, Ho and Er). Since a R.sub.2 Fe.sub.14 B phase composed
of a compound of a rare earth element produces monoaxial magnetic
anisotropy, R is an element effective in increasing coercive force (iHc),
and the R content is preferably in the range of 2 atomic % to 15 atomic %.
In order to maintain high magnetization without decreasing the Fe content,
and maintain a magnetic balance with coercive force (iHc), the R content.
is preferably in the range of 2 atomic % to 12 atomic %, more preferably
in the range of 2 atomic % to 8 atomic %.
M is at least one element selected from Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W
and Cu, which are elements effective in producing an amorphous phase, and
the M content is preferably in the range of 2 atomic % to 20 atomic %. In
order to obtain higher magnetic properties, the M content is preferably 2
atomic % to 15 atomic %, more preferably 2 atomic % to 6 atomic %. Of
these elements M, Cr is particularly effective. Cr may be partially
replaced by at least one element selected from Ti, Zr, Hf, V, Nb, Ta, Mo,
W and Cu. In replacement, with a composition ratio a in the range of
0.ltoreq.a.ltoreq.1, high .DELTA.Tx can be obtained. However, in order to
securely obtain particularly high .DELTA.Tx, the composition ratio a is
preferably in the range of 0.ltoreq.a.ltoreq.0.5. Of the elements M, Cu
has the effect of preventing the crystals from being coarsened in
crystallization for providing hard magnetism, and the function to improve
hard magnetic properties.
B has the high ability to form an amorphous phase, and the amount of B
added is 10 atomic % to 30 atomic %. With an amount of B added of less
than 10 atomic %, .DELTA.Tx undesirably disappears, and with an amount of
over 30 atomic %, the magnetic properties undesirably deteriorate. In
order to obtain the higher amorphous forming ability and good magnetic
properties, the amount of B added is preferably 14 atomic % to 20 atomic
%.
Furthermore, at least one element L selected from Ru, Rh, Pd, Os, Ir, Pt,
Al, Si, Ge, Ga, Sn, C and P may be added to the composition system.
In the present invention, at least one of these elements can be added in
the range of 0 to 5 atomic %. At least one of these elements is added for
mainly improving corrosion resistance. Out of the range, hard magnetic
properties deteriorate, and the glass forming ability undesirably
deteriorates.
In order to produce the hard magnetic alloy having the above composition
system, for example, single powders or single blocks (which may be
previously partly alloyed) of the respective elements are prepared and
then mixed so that the above composition ranges are obtained. Then the
mixed powders are melted in a melting device such as a crucible in an
atmosphere of inert gas such as Ar gas or the like to obtain an alloy melt
having the predetermined composition.
Then, the alloy melt is cast into a casting mold and slowly cooled or
quenched by a single roll method to obtain a glassy alloy. The
thus-obtained glassy alloy is subjected to heat treatment to precipitate
the predetermined crystalline phases. As a result, a bulk-shaped permanent
magnet molding thicker than a permanent magnet molding obtained from an
amorphous thin ribbon obtained by a conventional liquid quenching method
can easily be obtained without including rubber or a plastic. In this
method, the rate of temperature rise in heat treatment is 20.degree.
C./min or more.
The single roll method is a method comprising quenching a melt by spraying
onto a rotating metallic roll to obtain a thin ribbon of an amorphous
alloy.
In the thus-obtained bulk-shaped glassy alloy, heat treatment of a glassy
alloy having the above composition at a rate of temperature rise of
20C/min or more forms a mixed phase state comprising the soft magnetic
phase composed of the precipitated .alpha.-Fe phase and the hard magnetic
phase composed of the precipitated Nd.sub.2 Fe.sub.14 B phase. Also the
property of exchange coupling of the soft magnetic phase and the hard
magnetic phase can be improved to increase coercive force and maximum
magnetic energy product, thereby obtaining excellent hard magnetic
properties.
The bulk-shaped hard magnetic alloy contains no binder such as rubber,
plastic or the like, and thus has good magnetic properties and the
advantage of high material strength. The bulk-shaped hard magnetic alloy
also has excellent corrosion resistance and good rust resistance.
A sintered compact of the hard magnetic alloy in accordance with an
embodiment of the present invention is described with reference to the
drawings.
FIG. 1 shows a principal portion of a spark plasma sintering apparatus
suitable for producing a hard magnetic alloy sintered compact (referred to
as "the sintered compact of the present invention" hereinafter) of the
present invention. The spark plasma sintering apparatus comprises a
cylindrical die 1, an upper punch 2 and a lower punch 3 both of which are
inserted into the die 1, a punch electrode 4 which supports the lower
punch 3 and functions as one of electrodes for passing a pulse current
which will be described below, a punch electrode 5 which presses the upper
punch downward and functions as the other of the electrodes for passing
the pulse current, and a thermocouple 7 for measuring the temperature of a
raw powder 6 held between the upper and lower punches 2 and 3.
FIG. 3 shows the entire structure of the spark plasma sintering apparatus.
The spark plasma sintering apparatus A shown in FIG. 3 is a spark plasma
sintering machine, Model SPS-2050 made of Sumitomo Coal Mining Co., Ltd.,
and has a principal portion having the structure shown in FIG. 1.
The apparatus shown in FIG. 3 comprises an upper base 11 and a lower base
12, a chamber 13 in contact with the upper base 11, and the structure
shown in FIG. 1 most of which is contained in the chamber 13. The chamber
13 is connected to an evacuation device and an atmospheric gas supply
device, which are not shown in the drawing, so that the raw powder 6 held
between the upper and lower punches 2 and 3 can be maintained in a desired
atmosphere such as an inert gas atmosphere or the like. Although a current
carrying device is not shown in FIGS. 1 and 3, a current carrying device
is separately provided to be connected to the upper and lower punches 2
and 3 and the punch electrodes 4 and 5 so that the pulse current shown in
FIG. 2 can be supplied from the current carrying device through the
punches 2 and 3 and the punch electrodes 4 and 5.
In production of the sintered compact of the present invention by using the
spark plasma sintering apparatus having the above construction, the raw
powder for molding is first prepared. The raw powder is obtained by
melting a glassy alloy composition having the above predetermined
composition, forming a glassy alloy material having one of various shapes
such as bulk, ribbon, linear and powder shapes, and the like by a casting
method, a single roll or double roll quenching method, a liquid spinning
method, a solution extraction method, a high-pressure gas spraying method
or the like, and then powdering the glassy alloy material having a shape
other than a powder shape. The space between the upper and lower punches 2
and 3 of the die 1 is filled with the thus-obtained powder, for example,
the raw powder 6 shown in FIG. 1, and a pulse current is preferably
applied under pressure in the range of 200 to 1500 MPa to preferably
sinter the powder by heating to a temperature in the range of
Tx-250.ltoreq.Ts.ltoreq.Tx+250 at a rate of temperature rise of 40.degree.
C./min or more, to produce the sintered compact of the present invention.
The glassy alloy used in the present invention has a temperature width
.DELTA.Tx between the glass transition temperature Tg and the
crystallization temperature Tx. .DELTA.Tx is 20.degree. C. or more, and
some compositions has .DELTA.Tx of as large as 40.degree. C. or more or
60.degree. C. or more. Therefore, an amorphous material is basically
formed by solidification in the temperature range of .DELTA.Tx. By
applying appropriate heat and pressure to the amorphous powder by, for
example, a spark plasma sintering method, fine particles of the amorphous
powder are softened, with the surfaces fused with each other to obtain the
sintered compact. On the other hand, in heating and subsequent cooling,
for example, when the rare earth element R is Nd, a hard magnetic
crystalline phase such as the Nd.sub.2 Fe.sub.14 B phase is precipitated
in the amorphous powder fine particles, and magnetic anisotropy is
imparted to the softened amorphous matrix, followed by solidification to
form a hard magnetic sintered compact. Depending upon the sintering
temperature, a sintered compact is formed in an amorphous state, followed
by heat treatment at a temperature higher than the crystallization
temperature Tx to precipitate fine crystalline phases, for example, the
Nd.sub.2 Fe.sub.14 B phase, Fe phase and Fe.sub.3 B phase when the rare
earth element is Nd. The crystalline phases are made nanocomposite to form
a hard magnetic sintered compact.
Description will now be made of the method of sintering the thus-obtained
glassy alloy.
The glassy alloy obtained in any one of the above methods is ground, and
sufficiently mixed to obtain a uniform composition, to produce a raw
powder for sintering. The raw powder preferably has a particle diameter of
50 to 150 .mu.m. Then the space between the upper and lower punches 2 and
3 of the spark plasma sintering apparatus shown in FIG. 1 or 3 is filled
with the raw powder, and the inside of the chamber 13 is evacuated or
replaced with an inert gas. A pulse current, for example, as shown in FIG.
2, is applied to the raw powder under pressure applied from the upper and
lower punches 2 and 3, to mold the raw powder under heating.
The applied pressure is preferably in the range of 200 to 1500 MPa, more
preferably in the range of 500 to 1000 MPa. The thus-obtained hard
magnetic alloy sintered compact is a strong sintered compact having a fine
texture structure and used as a small strong permanent magnet having
rigidity as a physical property. The sintered compact preferably has a
relative density of 90% or more. Under the applied pressure of less than
200 MPa, it is difficult to impart anisotropy to the hard magnetic phase,
and the percentage of void of the obtained sintered compact is undesirably
increased to decrease the molding density. Under the applied pressure of
over 1500 MPa, the strength of the WC (tungsten carbide) die is
undesirably insufficient at high temperature.
In heating, the rate of temperature rise is 10.degree. C./min or more,
preferably 20.degree. C./min or more, more preferably 40.degree. C./min or
more. At a rate of temperature rise of less than 10.degree. C./min,
crystal grains are coarsened to deteriorate the hard magnetic properties.
In the spark plasma sintering method, if the crystallization temperature of
the amorphous glassy alloy is Tx, the sintering temperature Ts (.degree.
C.) is preferably in the range of Tx-250.ltoreq.Ts.ltoreq.Tx+250. At a
sintering temperature Ts of lower than Tx-250, it is difficult to form a
high-density sintered compact because the temperature is too low. At a
sintering temperature Ts of over Tx+250, the hard magnetic properties
undesirably deteriorate due to grain growth of the fine crystalline phase.
At a sintering temperature Ts in the above range, the system is softened,
and powder particles are densely compressed and fused with each other
under pressure to form a high-density bulk body according to the shape of
the mold used. At the same time, structural short-range ordering occurs in
the texture to produce and grow crystal nuclei having hard magnetism, such
as the Nd.sub.2 Fe.sub.14 B phase, and the produced hard magnetic fine
crystals form an exchange spring magnet with the soft magnetic crystalline
phase produced at the same time to cause magnetic anisotropy in the bulk
body. By imparting magnetic anisotropy to the crystal axis of the hard
magnetic phase, high remanent magnetization (Ir) is obtained, as compared
with the case of magnetic isotropy. Therefore, the sintered compact of the
present invention obtained after cooling comprises a strong bulk body
having a high density according to the shape of any desired mold, and
serves as a permanent magnet having improved coercive force and maximum
energy product.
In the above-mentioned spark plasma sintering, the temperature of the
entire raw powder can be uniformly increased at the predetermined rate,
and the temperature of the raw powder can be strictly controlled according
to the value of the current conducted. Therefore, the temperature can be
more precisely controlled than heating by a heater, and a high-density
sintered compact without a difference in the degree of sintering between
the outside and the inside can be obtained.
If required, the sintered compact obtained may be again subjected to heat
treatment in the range of 400 to 1000.degree. C. In some cases, this
causes precipitation of the hard magnetic fine crystalline phase having an
average crystal grain diameter of 100 nm or less at a higher density in
the sintered compact, thereby improving the hard magnetic properties.
Description will now be made of a cast magnet in accordance with an
embodiment of the present invention with reference to the drawings.
FIG. 4 shows an example of the casting apparatus used for producing a cast
magnet of the present invention. In FIG. 4, the casting apparatus
comprises a crucible 15 and a mold 17. The crucible 15 comprises a heating
high-frequency coil 14 provided on the periphery thereof so that a current
is passed through the high-frequency coil 14 to melt an alloy composition
16 of the present invention contained in the crucible 15 by heating. At
the lower end of the crucible 15 is formed a nozzle 15a, and the mold 17
made of copper or the like is arranged below the nozzle 15a. The mold 17
comprises a cylindrical casting cavity 18 formed therein.
Although not shown in the drawing, an inert gas supply device is connected
to the upper portion of the crucible 15 so that the inside of the crucible
15 can be maintained in an inert gas atmosphere, and if required, the
pressure in the crucible 15 can be increased to inject the melt of the
composition 16 into the casting cavity 18 of the mold 17 through the
nozzle 15a of the crucible 15.
In order to obtain a solidified product of the glassy alloy by using the
apparatus shown in FIG. 4, as shown in FIG. 5, predetermined pressure P is
applied to the inside of the crucible 15 to inject the melt into the
casting cavity 18 of the mold 17 through the nozzle 15a of the crucible
15, followed casting and then cooling of the cast melt. As a result, a
solidified product of the glassy alloy can be obtained.
The thus-obtained solidified product is removed from the mold, subjected to
heat treatment at a temperature in the range of 500 to 850.degree. C., and
then cooled to precipitate the soft magnetic phase and the hard magnetic
phase, thereby obtaining a cast magnet exhibiting exchange spring magnet
properties and having high Ir and iHc.
The glassy alloy composition used in the present invention has a
temperature width .DELTA.Tx between the crystallization temperature Tx and
the glass transition temperature Tg. Since .DELTA.Tx is 20.degree. C. or
more, and some compositions have .DELTA.Tx of as large as 40.degree. C. or
more or 60.degree. C. or more, the glassy alloy has the high ability to
form an amorphous phase, and a larger solidified product of the glassy
alloy can be formed. This molding is amorphous in the cast state, and thus
requires heat treatment accompanied with crystallization for providing
hard magnetism. The heat treatment is preferably carried out at a
temperature in the range of 500.degree. C. (773 K) to 850.degree. C. (923
K), and the heat treatment of the composition in this temperature range
causes precipitation of the soft magnetic phase composed of .alpha.-Fe and
Fe.sub.3 B and the hard magnetic phase composed of Nd.sub.2 Fe.sub.14 B to
exhibit exchange spring magnet properties and obtain high remanent
magnetization (Ir) and coercive force (iHc).
In heating for hat treatment, the rate of temperature rise is preferably
10.degree. C./min or more, more preferably 20.degree. C./min or more. At a
rate of temperature rise of less than 10.degree. C./min, crystal grains
are coarsened to decrease the exchange coupling force, thereby causing the
tendency that the hard magnetic properties deteriorate.
Although this embodiment relates to the casting apparatus comprising the
crucible 15 and the mold 17, the shapes of the crucible and the mold are
not limited to those described above. For example, such a casting
apparatus as shown in FIG. 6 may be used, in which a crucible-like melting
tank 21 comprises a cylinder 19 and a piston 20 provided as a crucible and
a mold at the bottom thereof so that the melt 16 is drawn into the
cylinder 19 and cooled by pulling down the piston 20. Of course, generally
used various casting apparatus can be used.
Description will now be made of a stepping motor comprising the hard
magnetic alloy in accordance with an embodiment of the present invention.
In order to produce a permanent magnet provided in the rotor of a stepping
motor by using the hard magnet alloy having the above composition system,
for example, single powders or single blocks (which may be previously
partially alloyed) of the respective elements are prepared, and then mixed
so that the above composition falls in the above ranges, and the mixed
powders are melted in a melting device such as a crucible or the like in
an inert gas atmosphere such as an Ar gas or the like to obtain an alloy
melt having the predetermined composition.
Next, the alloy melt is flowed into the casting mold to form a cast product
by the above casting method. After slow cooling, the product is subjected
to heat treatment to obtain a bulk plate of the hard magnetic glassy
alloy. Many bulk plates are laminated to obtain the permanent magnet used
for the rotor.
Alternatively, the melt is quenched by spraying it on a rotating roll,
followed by heat treatment to obtain a thin ribbon of the hard magnetic
glassy alloy having a thickness of 50 .mu.m or more. Many thin ribbons are
laminated to obtain the permanent magnet for the rotor.
The hard magnetic glassy alloy of the present invention has a supercooled
liquid region having a wide temperature width, and thus a thin ribbon
(plate material) having a thickness of 50 .mu.m or more can be obtained,
thereby decreasing the number of the thin ribbons (plate materials)
laminated.
The permanent magnet comprising the thus-obtained hard magnetic glassy
alloy exhibits good magnetic properties and high material strength because
no binder such as rubber, plastic or the like is included. Also the
permanent magnet has excellent corrosion resistance and good rust
resistance.
Furthermore, the permanent magnet of the rotor of the stepping motor can be
formed by grinding a bulk plate material of the hard magnetic glassy alloy
and then filling an appropriate mold with the powder obtained, followed by
sintering by the spark plasma sintering method.
Although the above sintered compact is formed by the spark plasma sintering
method, the forming method is not limited to this, and the permanent
magnet of the rotor can also be obtained by an extrusion molding method or
the like.
The rotor of the stepping motor comprises the hard magnetic alloy having a
supercooled liquid region having a temperature width .DELTA.Tx of
20.degree. C. or more, which is represented by the equation
.DELTA.Tx=Tx-Tg (wherein Tx is the crystallization temperature, and Tg is
the glass transition temperature), and thus a bulk shape can be obtained.
Therefore, the rotor can easily be formed in the predetermined shape,
thereby decreasing the production cost of the stepping motor.
Furthermore, there is no need for molding the material powder by sintering
at high temperature, thereby decreasing the production cost of the
stepping motor.
The hard magnetic glassy alloy can be molded to a predetermined shape by
flowing an alloy melt in a casing mold or the like, or sintering a raw
powder by a plasma. There is thus no need for using a binder such as
rubber, plastic or the like, thereby causing no deterioration in magnetic
properties and material strength. Therefore, even when the hard magnetic
glassy alloy of the present invention is used as a component material for
a driving unit such as the rotor of the stepping motor, there is no
problem of mechanical strength.
Also, even when ribbons of the hard magnetic alloy obtained by the single
roll method are laminated to form the permanent magnet, part of the hard
magnetic alloy is amorphous and has high hardness. Therefore, even when
the hard magnetic glassy alloy of the present invention is used as a
component material for a driving unit such as the rotor of the stepping
motor, there is no problem of mechanical strength.
In the hard magnetic alloy, a mixed phase state is formed by heat treatment
in which the soft magnetic phase composed of the precipitated .alpha.-Fe
phase, and the hard magnetic phase composed of the precipitated Nd.sub.2
Fe.sub.14 B phase. Therefore, the hard magnetic alloy has the exchange
spring magnet properties in which the soft magnetic phase and the hard
magnetic phase are magnetically coupled, and thus the maximum magnetic
energy product ((BH.sub.max)) can be increased, thereby increasing the
rotational torque of the stepping motor.
Also the hard magnetic alloy exhibits high coercive force (iHc), and thus
permits miniaturization of the stepping motor.
The construction of the stepping motor is described.
The stepping motor is a special motor in which the amount of rotation can
be arbitrarily controlled by a pulse current. Therefore, the stepping
motor requires no feedback control, is capable of positioning in an open
loop, and can thus be used as a driving source in a positioning system in
various fields. Particularly, since a hybrid stepping motor exhibits high
rotational torque and is small and capable of performing precise
positioning control, the stepping motor is used as a driving source of a
driving mechanism in a copying machine, a computer, or the like.
The hybrid type stepping motor (referred to as "the HB type motor"
hereinafter) is described with reference to the drawings.
Referring FIG. 7, a HB type motor 22 comprises a stator 24 comprising a
stator 23, and a rotor 25.
The stator 23 has a plurality of stator tooth poles 26 formed in the
surface on the rotor side thereof. The stator 23 is made of a soft
magnetic material having high magnetic permeability, and a lead wire 27 is
wound on the stator 23. The stator 23 and the lead wire 27 constitute an
electromagnet. The lead wire 27 is connected to a driving circuit not
shown in the drawing.
By alternately passing currents having opposite polarities through the lead
wire 27 from the driving circuit, the stator tooth poles 26 on the surface
on the rotor side are alternately magnetized to the N and S poles.
In FIGS. 7 and 8A, the rotor 25 comprises a shaft 28, a cylindrical hard
magnetic alloy 29 into which the shaft 28 is inserted, and two pairs of
cylindrical rotors 30 and 31 which are fit to both ends of the hard
magnetic alloy 29.
The hard magnetic alloy 29 is arranged so that the direction of the
magnetic flux is parallel to the length direction of the shaft 28. The
rotors 30 and 31 are made of a soft magnetic material having high magnetic
permeability, the rotor 30 being magnetized to the N pole, the other 31
being magnetized to the S pole.
The rotor 30 has a plurality of tooth poles 32 formed in the surface on the
stator side thereof, and the rotor 31 has a plurality of tooth poles 33
formed in the surface on the stator side thereof.
As shown in FIG. 8B, the rotor tooth poles 32 and the rotor tooth poles 33
are arranged at both ends of the hard magnetic alloy 29 so that the
troughs 34 and the crests 35 of one of the rotors are shifted from the
other rotor in the length direction of the shaft 28.
The operation of the HB type motor 22 is described.
In FIGS. 9A and 9B, the stator tooth poles 26a of a stator 23a are
magnetized to the N pole by the current passed through the lead 27 from
the driving circuit, the stator tooth poles 26b of a stator 23b are
magnetized to the S pole.
Since the rotor tooth poles 32 of the rotor 30 are magnetized to the N
pole, magnetic fluxes between the rotor tooth poles 32 and the stator
tooth poles 26a are cancelled, and magnetic fluxes between the rotor tooth
poles 32 and the stator tooth poles 26b are intensified.
Also, since the rotor tooth poles 33 of the rotor 31 are magnetized to the
S pole, magnetic fluxes between the rotor tooth poles 33 and the stator
tooth poles 26a are intensified, and magnetic fluxes between the rotor
tooth poles 33 and the stator tooth poles 26b are cancelled.
Since the rotor tooth poles 32 and the other rotor tooth poles 33 are
arranged so that the troughs 34 and the crests 35 of one rotor are shifted
from the other rotor, the rotor tooth poles 32 and the stator tooth poles
26b, and the rotor tooth poles 33 and the stator tooth poles 26a attract
each other to rotate the rotors 30 and 31 by an amount corresponding to
one crest 35 of the rotor tooth poles 32 and 33, respectively, in the
direction shown by arrows in the drawing.
Next, in FIGS. 10A and 10B, by changing the polarity of the current from
the control circuit, the stator tooth poles 26a are magnetized to the S
pole, and the stator tooth poles 26b are magnetized to the N pole. In this
case, magnetic fluxes are intensified between the rotor tooth poles 32 and
the stator tooth poles 26a, and magnetic fluxes are cancelled between the
rotor tooth poles 32 and the stator tooth poles 26b.
Also, magnetic fluxes are cancelled between the rotor tooth poles 33 and
the stator tooth poles 26a, and magnetic fluxes are intensified between
the rotor tooth poles 33 and the stator tooth poles 26b.
Therefore, the rotor tooth poles 32 and the stator tooth poles 26a, and the
rotor tooth poles 33 and the stator tooth poles 26b attract each other to
rotate the rotors 30 and 31 by an amount corresponding to one crest 35 of
the rotor tooth poles 32 and 33, respectively, in the left-hand direction
shown in the drawing.
In this way, by passing the current through the HB type motor 22 from the
driving circuit, the rotor 25 can be driven for a predetermined rotation
angle.
The properties of the hard magnetic alloy are represented by the second
quadrants of hysteresis curves, i.e., demagnetization curves. After
magnetization, the hard magnetic alloy is under the reverse magnetic
field, i.e., the diamagnetic field, produced by its remanent
magnetization, and thus the operating point (the magnetic flux density (B)
and demagnetizing field (H) of a material) is represented by a point p on
the demagnetization curve thereof. At this point, the product (BH)
represents the maximum energy product ((BH).sub.max).
In order to increase the rotational torque of the stepping motor, it is
important to use the hard magnetic alloy having the high maximum energy
product ((BH).sub.max).
Since the rotational torque of the stepping motor is proportional to the
product of the current passing through the stepping motor and the energy
(U) of the magnetostatic field produced outside by a hard magnetic alloy,
the rotational torque of the stepping motor is increased by increasing the
maximum energy product ((BH).sub.ma x).
In order to increase the maximum energy product ((BH).sub.max) of a hard
magnetic alloy, it is necessary to made the shape of a demagnetization
curve angular to increase the area surrounded by the demagnetization
curve, the magnetic field axis and the magnetization axis. Namely, it is
necessary to increase the remanence ratio (Ir/Is) to increase remanent
magnetization (Ir) and coercive force (iHc).
Description will now be made of a speaker in accordance with an embodiment
of the present invention with reference to the drawings.
FIG. 11 is a sectional view showing a speaker in accordance with a first
embodiment of the present invention.
In FIG. 11, a speaker comprises a pole piece 36 made of iron, a cylindrical
dust core (yoke) provided on the outside of the pole piece 36 with a space
therebetween, and upper and lower speaker magnets 38 and 39 arranged in
the space between the pole piece 36 and the yoke 37, and a conical
diaphragm 40. The speaker magnets 38 and 39 are made of the hard magnetic
alloy sintered compact or cast magnet of the present invention, and formed
in a ring shape. Also, a voice coil (not shown in the drawing) is arranged
in the magnetic gap formed by the speaker magnets 38 and 39, and is
connected to the conical diaphragm 40. In the speaker constructed as
described above, when a voice current flows through the voice coil from an
amplifier, motion is accordingly caused to move the conical diaphragm 40
connected to the voice coil so that sounds can be emitted.
FIG. 12 is a sectional view showing a speaker in accordance with a second
embodiment of the present invention.
In FIG. 12, the speaker comprises a pair of upper and lower pole pieces 41
and 42 made of iron and arranged opposite to each other, a speaker magnet
43 disposed between the pole pieces 41 and 42, a cylindrical yoke 44
provided on the outside of the pole pieces 41 and 42 and the speaker
magnet 43 with a space therebetween, a conical diaphragm 45, and a
magnetic shielding cover 46. The speaker magnet 43 comprises the hard
magnetic glassy alloy sintered compact, and is formed in a ring shape. The
pole pieces 41 and 42, and the magnet 43 are mounted to the magnetic
shielding cover 46 by means of a bolt 47, a washer 48, and a nut 49.
The speaker of the second embodiment comprises the speaker magnet 43
comprising the glassy alloy sintered compact, and thus has substantially
the same effect as the speaker of the first embodiment.
EXAMPLES
Production Example 1
Single pure metals of Fe, Co, Nd, Cr and Zr and a pure boron crystals were
mixed in an Ar gas atmosphere, and melted by an arc to produce a master
alloy.
Next, the master alloy was melted in a crucible, and then quenched by the
single roll method in which the melt was sprayed, at an injection pressure
of 0.50 kgf/cm.sup.2, onto a copper roll rotating at a rate of 4000 rpm
through a nozzle having a diameter of 0.35 to 0.45 mm formed at the lower
end of the crucible in an argon gas atmosphere at 60 cmHg to produce metal
alloy ribbon samples having a width of 0.4 to 1 mm and a thickness of 20
to 30 .mu.m. In the single roll liquid quenching apparatus used, the
surface of the single roll was finished to #1500. The gap between the
single roll and the nozzle tip was 0.30 mm.
The obtained samples were analyzed by X-ray diffraction and differential
scanning calorimetry (DSC), observed on a transmission electronic
microscope (TEM), and measured with respect to magnetic properties at room
temperature and 15 kOe by a vibrating sample type magnetometer (VSM).
FIG. 13 shows the results of measurement of DSC curves of samples having
the composition Fe.sub.63 Co.sub.7 Nd.sub.10-x Zr.sub.x B.sub.20 (x=0, 2,
4 and 6) when heated in the range of 127 to 827.degree. C. at a rate of
temperature rise of 0.67.degree. C./sec.
FIG. 13 indicates that in the sample having the composition Fe.sub.63
Co.sub.7 Nd.sub.10- B.sub.20, at least three exothermic peaks are
observed, and thus crystallization possibly takes place in at least three
steps. It is also found that the glass transition temperature Tg is
observed below the crystallization temperature Tx, but with an amount of
Zr added of 4 atomic % or more, endothermic reaction possibly
corresponding to Tg is observed at a temperature below Tg.
FIG. 14 is a chart showing the result of X-ray diffraction analysis of the
glassy alloy ribbon sample having the composition Fe.sub.63 Co.sub.7
Nd.sub.6 Zr.sub.4 B.sub.20 shown in FIG. 13 after annealing at 560.degree.
C. (833 K) for 300 seconds immediately after the temperature of
endothermic reaction, and the result of X-ray diffraction analysis of the
glassy alloy ribbon sample having the composition Fe.sub.63 Co.sub.7
Nd.sub.4 Zr.sub.6 B.sub.20 shown in FIG. 13 after annealing at 570.degree.
C. (843 K) for 300 seconds immediately after the temperature of
endothermic reaction.
FIG. 14 reveals that in both glassy alloy ribbon samples respectively
having the compositions Fe.sub.63 Co.sub.7 Nd.sub.6 Zr.sub.4 B.sub.20 and
Fe.sub.63 Co.sub.7 Nd.sub.4 Zr.sub.6 B.sub.20, no diffraction peak
corresponding to crystallization is observed, and only a broad peak is
observed near 2.theta.=45.degree.. It is thus found that endothermic
reaction observed below the crystallization temperature Tx is endothermic
reaction corresponding to glass transition.
This indicates that at a Zr content of 4 atomic %, the temperature width
.DELTA.Tx (=Tx-Tg) of the supercooled liquid region is .DELTA.Tx=30 K, and
at a Zr content of 6 atomic %, the temperature width .DELTA.Tx is
.DELTA.Tx=35 K, and that the higher the Zr content is, the wider the
temperature width .DELTA.Tx of the supercooled liquid region is.
FIG. 15 is a X-ray diffraction chart of glassy alloy ribbon samples having
the compositions Fe.sub.63 Co.sub.7 Nd.sub.8 Cr.sub.2 B.sub.20, Fe.sub.63
Co.sub.7 Nd.sub.6 Cr.sub.4 B.sub.20 and Fe.sub.63 Co.sub.1 Nd.sub.4
Cr.sub.6 B.sub.20. As a comparative example, a glassy alloy ribbon sample
of Fe.sub.63 Co.sub.7 Nd.sub.10 B.sub.20 (containing no metal M) prepared
by the same method as described above was also subjected to X-ray
diffraction analysis. X-ray diffraction analysis was carried out by using
an X-ray diffractometer (XRD) using Cu-K.alpha. rays.
The results shown in FIG. 16 reveal that all patterns obtained are typical
broad patterns, and thus all samples are amorphous.
Next, samples having the composition Fe.sub.63 Co.sub.7 Nd.sub.10-x
Cr.sub.x B.sub.20 (x=0, 2, 4 and 6) were sealed in a vacuum, and then
subjected to heat treatment at 560.degree. C. (833 K) to 900.degree. C.
(1173 K) for a holding time of 300 seconds by using a muffle furnace to
measure the dependence of magnetic properties on the heat treatment
temperature. FIG. 16 shows the results of measurement.
The results shown in FIG. 16 indicate that in the samples (x=2, 4 and 6)
containing Cr, the value of saturation magnetization is as high as 1 T or
more, which is higher than the comparative sample (x=0) containing no Cr.
All samples show the tendency that remanent magnetization increases as the
heat treatment temperature increases. The samples (x=2, 4 and 6)
containing Cr show a large increase in remanent magnetization to about 0.8
T, and a high remanence ratio, as compared with the comparative sample
(x=0) containing no Cr. The samples (x=2, 4 and 6) containing Cr exhibit
lower coercive force than the comparative sample (x=0) containing no Cr
regardless of the amount of Cr added and the heat treatment temperature,
but the sample of x=4 shows a high maximum energy product.
Next, a sample having the composition Fe.sub.63 Co.sub.7 Nd.sub.6 Cr.sub.4
B.sub.20 was sealed in a vacuum, and then subjected to heat treatment at
650.degree. C. (923 K) for a holding time of 300 seconds at a heating rate
of 10.degree. C./min or more by using a muffle furnace to measure the
dependence of magnetic properties on heating rate. The results of
measurement are shown in FIG. 17 and Table 1.
Table 1 also shows the density of a glassy alloy ribbon sample after
quenching in production by the single roll method.
TABLE 1
Dependence of magnetic properties of Fe.sub.63 Co.sub.7 Nd.sub.6 Cr.sub.4
B.sub.20 on
rate of heating rate
a Is Ir iHc (BH).sub.max Density
(.degree. C./min) (T) (T) Ir/Is (kA/m) (kJ/m.sup.3) (10.sup.3
kg/m.sup.3)
as-Q 0.904 0.079 0.088 -- -- 6.774
10 1.005 0.713 0.709 321.97 39.63
20 1.008 0.722 0.716 340.03 42.46
40 0.986 0.738 0.748 399.08 53.83
80 0.985 0.757 0.769 453.67 64.48
.infin. 0.989 0.740 0.748 410.06 56.98
In Table 1, as-Q represents the alloy ribbon sample after quenching without
heat treatment, a represents the rate of temperature rise in heat
treatment, m represents the maximum of the heating rate, Is represents
saturation magnetization, Ir represents remanent magnetization, Ir/Is
represents remanence ratio, iHc represents coercive force, and
(BH).sub.max represents the maximum magnetic energy product.
The results shown in FIG. 17 and Table 1 reveal that in heat treatment of
the glassy alloy ribbon sample having the composition Fe.sub.63 Co.sub.7
Nd.sub.6 Cr.sub.4 B.sub.20, at a heating rate of 20.degree. C./min or
more, saturation magnetization and remanent magnetization hardly change,
but coercive force and maximum magnetic energy product tend to increase,
at a heating rate of 40.degree. C./min or more, a coercive force of about
400 kA/m and a maximum magnetic energy product of 54 kJ/m.sup.3 are
obtained, and at a rate of temperature rise of 80.degree. C./min or more,
a coercive force of about 450 kA/m and a maximum magnetic energy product
of 65 kJ/m.sup.3 are obtained. At a heating rate close to 80.degree.
C./min or more, coercive force and maximum magnetic energy product are
maximum, and the magnetic properties adversely deteriorate even if the
heating rate is increased to 80.degree. C./min or more. Therefore, for the
glassy alloy having the above composition, the upper limit of the heating
rate temperature rise is possibly about 80.degree. C./min. FIGS. 18 to 20
show I-H loops before and after heat treatment with respect to glassy
alloy ribbon samples respectively having the compositions Fe.sub.63
Co.sub.7 Nd.sub.8 Cr.sub.2 B.sub.20, Fe.sub.63 Co.sub.7 Nd.sub.6 Cr.sub.4
B.sub.20 and Fe.sub.63 Co.sub.7 Nd.sub.4 Cr.sub.6 B.sub.20.
FIG. 21 shows a I-H loop before and after heat treatment with respect to a
comparative sample having the composition Fe.sub.63 Co.sub.7 Nd.sub.10
B.sub.20.
FIGS. 18 to 20 indicate that the amorphous alloy ribbon sample as a
comparative example having the composition Fe.sub.63 Co.sub.7 Nd.sub.10
B.sub.20 after quenching without heat treatment shows soft magnetism, and
hard magnetism is exhibited after crystallization heat treatment. It is
also found that since the deposited phase is very fine in the early stage
of crystallization, and as the heat treatment temperature increases, the
coercive force decreases, and the remanence ratio deteriorates. This
indicates that grain growth of each deposited phase, particularly the soft
magnetic phase, occurs with an increase in the heat treatment temperature.
On the other hand, the glassy alloy ribbon sample containing 2 to 6 atomic
% of Cr as each of examples after quenching without heat treatment shows
soft magnetism, and hard magnetic is exhibited after crystallization heat
treatment. It is also found that in the examples, saturation magnetization
and remanent magnetization are very high, and coercive force increases in
the early stage of crystallization, becomes maximum after first
crystallization, and then slightly decreases. This indicates that the
maximum energy product is higher than the comparative example, and that
the glassy alloy ribbon sample of each of the examples can be used as an
exchange spring magnet comprising the soft magnetic phase and the hard
magnetic phase.
Production Example 2
Single pure metals of Fe, Co, Nd and Cr and pure boron crystals were mixed
in an Ar gas atmosphere and melted by an arc to produce a master alloy.
The master alloy was melted in a crucible, and then quenched by the same
single roll method as Production Example 1 to produce glassy alloy ribbon
samples having a width of 0.4 to 1 mm and a thickness of 20 to 30 .mu.m.
The thus-obtained samples were analyzed by X-ray diffraction and
differential scanning calorimetry (DSC), observed on a transmission type
electron microscope (TEM) and measured with respect to magnetic properties
by a vibrating sample type magnetometer (VSM) at room temperature and 15
kOe.
Next, the produced glassy alloy ribbon samples having the composition
Fe.sub.63 Co.sub.7 Nd.sub.10-x Cr.sub.x B.sub.20 (x=0, 2, 4 and 6) were
sealed in a vacuum, and then subjected to heat treatment at 585.degree. C.
(858 K) to 750.degree. C. (1023 K) for a holding time of 300 seconds by
using a muffle furnace to measure the dependence of magnetic properties on
the heat treatment temperature. Table 2 shows the results of measurement.
Table 2 also shows the density of a glassy alloy ribbon sample having each
of the compositions after quenching in production by the sample roll
method.
TABLE 2
Magnetic properties of Fe.sub.63 Co.sub.7 Nd.sub.10-x Cr.sub.x B.sub.20
Heat treatment Is Ir iHc (BH).sub.max
Density
temperature (T) (T) Ir/Is (KA/m) (kJ/m.sup.3)
(10.sup.3 kg/m.sup.3)
Fe.sub.63 Co.sub.7 Nd.sub.10 B.sub.20 as-Q 0.964 0.064 0.066
49.66 0.08 6.510
660 0.287 0.169 0.588 1051.22 11.73
670 0.349 0.232 0.664 1037.69 14.32
680 0.408 0.296 0.725 848.30 13.01
690 0.459 0.341 0.743 663.91 21.00
700 0.477 0.358 0.750 633.91 22.99
750 0.630 0.401 0.637 289.10 17.71
Fe.sub.63 Co.sub.7 Nd.sub.8 Cr.sub.2 B.sub.20 as-Q 0.933 0.083
0.084 -- -- 6.771
645 1.194 0.931 0.780 5.51 2.93
665 1.177 0.908 0.772 6.22 3.34
700 1.180 0.893 0.757 7.12 3.80
750 1.028 0.613 0.596 7.52 5.98
Fe.sub.63 Co.sub.7 Nd.sub.6 Cr.sub.4 B.sub.20 as-Q 0.904 0.079
0.088 -- -- 6.774
620 1.038 0.742 0.715 242.63 31.00
640 1.018 0.737 0.723 307.17 39.09
650 0.989 0.740 0.748 410.06 56.98
700 0.976 0.727 0.745 394.70 51.36
Fe.sub.63 Co.sub.7 Nd.sub.4 Cr.sub.6 B.sub.20 as-Q 0.864 0.069
0.080 -- -- 6.777
585 0.979 0.825 0.843 144.99 42.84
600 0.964 0.818 0.848 197.11 56.23
650 0.969 0.763 0.788 224.17 46.04
In Table 2, as-Q represents an alloy ribbon sample after quenching without
heat treatment, Ta represents the heat treatment temperature, Is
represents saturation magnetization, Ir represents remanent magnetization,
Ir/Is represents remanence ratio, iHc represents coercive force, and
(BH).sub.max represents the maximum magnetic energy product.
The results shown in Table 2 indicate that in the example samples
containing Cr, the value of saturation magnetization is as high as 1 T or
more, which is higher than the comparative sample not containing Cr. The
samples containing Cr show a large increase in remanent magnetization to
about 0.6 to 0.9 T, and a high remanence ratio, as compared with the
comparative sample not containing Cr.
Next, the sample having each of the compositions shown in Table 2 was
heated in the range of 127 to 827.degree. C. at a heating rate of
0.67.degree. C./sec to examine the temperature width .DELTA.Tx of the
supercooled liquid region from a DSC curve. As a result, in the amorphous
alloy ribbon sample of a comparative example having the composition
Fe.sub.63 Co.sub.7 Nd.sub.10 B.sub.20, .DELTA.Tx is not observed, while
the glassy alloy ribbon samples having the compositions Fe.sub.63 Co.sub.7
Nd.sub.8 Cr.sub.2 B.sub.20 Fe.sub.63 Co.sub.7 Nd.sub.6 Cr.sub.4 B.sub.20,
Fe.sub.63 Co.sub.7 Nd.sub.4 Cr.sub.6 B.sub.20 and Fe.sub.63 Co.sub.7
Nd.sub.4 Zr.sub.6 B.sub.20 exhibit .DELTA.Tx of 51.degree. C., 40.degree.
C., 52.degree. C., and 25.degree. C., respectively. It is thus found that
a sample containing Cr exhibits a supercooled liquid region having a wider
temperature width .DELTA.Tx.
Production Example 3
Description will be made of an example of production of the hard magnetic
alloy sintered compact of the present invention.
An amorphous glassy alloy having each of the compositions shown in Table 3
was prepared. First, an alloy ingot having each of the compositions was
prepared by an arc melting method, and then the resultant melt was sprayed
on a rotating Cu roll in an Ar atmosphere to obtain a quenched ribbon
having a thickness of about 20 .mu.m. The thus-obtained quenched ribbon
was ground by suing a rotor speed mill to produce an amorphous glassy
alloy powder having a particle size of 50 to 150 .mu.m.
A sintered compact was formed by the spark plasma sintering apparatus shown
in FIG. 1 using each of the obtained various glassy alloy powders by the
method described below. The inside of a WC die was filled with about 2 g
of each glassy alloy powder by using a hand press, and inserted into the
die 1 of the spark plasma sintering apparatus shown in FIG. 1. The inside
of the chamber was pressurized by the upper and lower punches 2 and 3 in
an atmosphere of x 10.sup.-5 torr, and at the same time, a pulse current
was applied to the raw powder from the current carrying device to heat the
powder. The pulse waveform comprised 12 pulses conducted and 2 pulses in a
quiescent time, as shown in FIG. 2 so that the raw powder was heated at a
current of 4700 to 4800 A at maximum.
Sintering was carried out by heating a sample from room temperature to the
sintering temperature Ts (*C) shown in Table 3 with the pressure Ps (MPa)
shown in Table 3 applied to the sample, and holding the sample at this
temperature for about 5 minutes. The heating rate was 100.degree. C./min.
Each of the obtained sintered compacts was measured with respect to
magnetic properties including saturation magnetization Is (T), remanent
magnetization Ir (T), remanence ratio Ir/Is, coercive force iHc (kOe), and
maximum energy product (BH).sub.max (kJ/m.sup.3), and the relative density
(%).
In regard to the magnetic properties, properties in the three axis
directions X, Y and Z wherein the Z axis is the direction of application
of sintering pressure Ps in sintering, and the X and Y axes are
perpendicular to the Z axis. The remanent magnetization Ir (T) is a value
represented by the following equation:
Ir (T)=4.pi..times.7.0.times.relative density.times.Ir (emu/g)/10000 The
relative density (%) is a value relative to the true density (about 7.0
g/cm.sup.3). The results are shown in Table 3.
TABLE 3
Alloy composition Is Ir iHc (BH)max
Relative Density
and sintering condition Direction (T) (T) (kA/m) (kJm.sup.-3) Ir/Is
(%)
Fe.sub.63 Co.sub.7 Nd.sub.8 Cr.sub.2 B.sub.20 X 1.03 0.61 77.5
6.0 0.59 93
Ts = 750.degree. C. Y 1.03 0.61 77.5 6.0 0.59
Ps = 633 MPa Z 1.20 0.88 93.0 7.2 0.73
Fe.sub.63 Co.sub.7 Nd.sub.6 Cr.sub.4 B.sub.20 X 0.99 0.74 410
60 0.75 94
Ts = 650.degree. C. Y 0.99 0.74 410 60 0.75
Ps = 633 MPa Z 1.19 1.07 490 72 0.90
Fe.sub.63 Co.sub.7 Nd.sub.4 Cr.sub.6 B.sub.20 X 0.97 0.76 224
46 0.78 94
Ts = 600.degree. C. Y 0.97 0.76 224 46 0.78
Ps = 633 MPa Z 1.15 1.09 269 55 0.94
Ts = sintering temperature
Ps = sintering pressure
The results shown in Table 3 indicate that all sintered compacts obtained
by the spark plasma sintering method using the alloy compositions shown in
Table 3 have excellent hard magnetic properties. It is also found that in
all samples, Is, Ir and Ir/Is are relatively high in the Z direction, and
(BH).sub.max is high. This indicates that the hard magnetic phase
crystallized or experienced grain growth by sintering under stress is made
anisotropic, thereby imparting magnetic anisotropy.
Next, for a glassy alloy ribbon sample having the composition Fe.sub.63
Co.sub.7 Nd.sub.6 Zr.sub.4 B.sub.20, the relation between the heating
temperature (.degree. C.) and the amount of heat generated was examined.
The results of measurement are shown in FIG. 22. FIG. 22 shows a DSC curve
of a glassy alloy ribbon sample having the composition Fe.sub.63 Co.sub.7
Nd.sub.6 Zr.sub.4 B.sub.20.
For a glassy alloy ribbon sample having the composition Fe.sub.63 Co.sub.7
Nd.sub.6 Zr.sub.4 B.sub.20, the relation between the heating temperature
(.degree. C.) and the elongation percentage was also examined. The results
of measurement are shown in FIG. 23. In FIG. 23, curves (i) and (ii) are a
thermal mechanical analysis (TMA) curve and a delta thermal mechanical
analysis (DTMA) curve, respectively, of the glassy alloy ribbon sample
having the composition Fe.sub.63 Co.sub.7 Nd.sub.6 Zr.sub.4 B.sub.20.
In FIGS. 22 and 23, the DSC curve shows exothermic peaks at about
647.degree. C. and 687.degree. C., the DTMA curve indicates that the
absolute differential value is high at about 627.degree. C. and the sample
is easily elongated at about 627.degree. C., and the TMA curve indicates
that the sample is rapidly elongated with an increase in temperature in
the region of 577.degree. C. to 677.degree. C. This indicates that
softening of the alloy takes place in the supercooled liquid region.
Therefore, it is possible to obtain a sintered compact with a high density
by solidification molding which employs such softening of an amorphous
alloy, and obtain a sintered compact having excellent hard magnetic
properties such as remanent magnetization (Ir), coercive force (iHc),
maximum magnetic energy product ((BH).sub.max), etc.
As described above, the hard magnetic glassy alloy of this embodiment has a
high density and good hard magnetic properties such as remanent
magnetization (Ir), coercive force (iHc), maximum magnetic energy product
((BH).sub.max), etc. Therefore, by using such a hard magnetic glassy alloy
for the stepping motor, it is possible to miniaturize the stepping motor
and increase the rotational torque.
The technical field of the present invention is not limited to the above
embodiments, various changes can be made in the scope of the gist of the
present invention.
Although, in the above embodiments, the present invention is applied to the
hybrid stepping motor, the present invention can also be applied to a
permanent magnet type stepping motor. Although the above embodiments
relate to the rotation type motor, the present invention can also be
applied to a linear motor.
Production Example 4
Description will be made of an example in which the hard magnetic ally of
the present invention was cast.
Single pure metals of Fe, Co, Nd and Cr, and pure boron crystals were mixed
in an Ar gas atmosphere, and then melted by an arc to produce a master
alloy having the following composition:
Fe.sub.63 Co.sub.7 Nd.sub.6 Cr.sub.4 B.sub.20
Next, the master alloy was ground, and 5 g of the powder was put in the
crucible 15 of the casting apparatus shown in FIG. 4. A current was passed
through the high-frequency coil 14 to melt the master alloy by heating at
a temperature 100 to 200.degree. C. higher than the melting point of the
master alloy in an inert gas atmosphere. The thus-obtained melt 16 was
injected into the casting mold 17 having the casting cavity 18 of .O
slashed.1 mm.times.50 mm formed therein, at an injection pressure of 0.5
to 1.5 kgf/cm.sup.2 through the nozzle 15a having a pore diameter of 0.5
to 0.6 mm and provided at the bottom of the crucible to obtain a
solidified molding of .O slashed.1 mm.times.50 mm.
The thus-obtained molding was measured by X-ray diffraction analysis by
using a X-ray diffractometer (XRD) using Cu-K.alpha. rays. As a result,
the pattern was a typical broad pattern, and thus it was confirmed that
the molding comprises an amorphous phase. Also the results of DSC
measurement shown in FIG. 24 show the following.
Crystallization temperature Tx=655.degree. C.
Glass transition temperature Tg=614.degree. C.
It was thus confirmed that the molding shows the following:
Supermodel liquid region .DELTA.Tx=41.degree. C.
Next, the molding was sealed in a quartz tube under vacuum, and then
subjected to heat treatment by an electric furnace under the following
conditions:
Heating rate: 80.degree. C./min.
Heat treatment temperature: 620 to 700.degree. C. (893 to 973 K)
Holding time: 5 minutes
Cooling: water quenching
The molding was crystallized by heat treatment to obtain a cast magnet
comprising a Fe.sub.3 B phase as a soft magnetic phase and a Nd.sub.2
Fe.sub.14 B phase as a hard magnetic phase, which were precipitated in an
amorphous matrix. Table 4 shows the magnetic properties of the cast
magnet.
TABLE 4
Heat treatment
temperature Is Ir iHc (BH).sub.max
Density
Composition (.degree. C.) (T) (T) Ir/Is (KA/m)
(kJm.sup.3) (g/cm.sup.3)
Fe.sub.63 Co.sub.7 Nd.sub.6 Cr.sub.4 B.sub.20 as-Q 0.99 0.08
0.09 -- -- 6.7
620 1.04 0.74 0.72 243 31.0
640 1.02 0.74 0.72 307 39.1
650 0.99 0.74 0.75 410 57.0
700 0.98 0.73 0.75 395 51.4
As described above in the embodiments, the glassy alloy composition can be
formed in a molding having any desired shape, and heat treatment of the
molding permits formation of a cast magnet having high performance and
comprising the soft and hard magnetic fine crystalline phases which are
precipitated in an amorphous matrix. By using the sintered magnet or the
cast magnet as the core of a stepping motor and a speaker, it is possible
to easily provide a stepping motor or a speaker having excellent
properties.
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